Facet

Service manager for asyncio.

Reason

mode tries to do too much job:

• Messy callbacks (on_start, on_started, on_crashed, etc.).
• Inheritance restrict naming and forces super() calls.
• Forced logging module and logging configuration.

Features

• Simple (start, stop, dependencies and add_task).
• Configurable via inheritance (graceful shutdown timeout).
• Mixin (no super() required).
• Requires no runner engine (Worker, Runner, etc.) just plain await or async with.

License

facet is offered under MIT license.

Requirements

• python 3.6+

Usage

import asyncio
import logging

from facet import ServiceMixin


class B(ServiceMixin):

    def __init__(self):
        self.value = 0

    async def start(self):
        self.value += 1
        logging.info("b started")

    async def stop(self):
        self.value -= 1
        logging.info("b stopped")


class A(ServiceMixin):

    def __init__(self):
        self.b = B()

    @property
    def dependencies(self):
        return [self.b]

    async def start(self):
        logging.info("a started")

    async def stop(self):
        logging.info("a stopped")


logging.basicConfig(level=logging.DEBUG)
asyncio.run(A().run())

This will produce:

INFO:root:b started
INFO:root:a started

Start and stop order determined by strict rule: dependencies must be started first and stopped last. That is why B starts before A. Since A may use B in start routine.

Hit ctrl-c and you will see:

INFO:root:a stopped
INFO:root:b stopped
Traceback (most recent call last):
  ...
KeyboardInterrupt

Stop order is reversed, since A may use B in stop routine. Any raised exception propagates to upper context. facet do not trying to be too smart.

Service can be used as a context manager. Instead of

asyncio.run(A().run())

Code can look like:

async def main():
    async with A() as a:
        assert a.b.value == 1
        await a.wait()

asyncio.run(main())

Another service feature is add_task method:

class A(ServiceMixin):

    async def task(self):
        await asyncio.sleep(1)
        logging.info("task done")

    async def start(self):
        self.add_task(self.task())
        logging.info("start done")


logging.basicConfig(level=logging.DEBUG)
asyncio.run(A().run())

This will lead to background task creation and handling:

INFO:root:start done
INFO:root:task done

Any non-handled exception on background task will lead the whole service stack crashed. This is also a key feature to fall down fast and loud.

All background tasks will be cancelled and awaited on service stop.

You can manage dependencies start/stop to start sequently, parallel or mixed. Like this:

class A(ServiceMixin):

    def __init__(self):
        self.b = B()
        self.c = C()
        self.d = D()

    @property
    def dependencies(self):
        return [
            [self.b, self.c],
            self.d,
        ]

This leads to first b and c starts parallel, after they successfully started d will try to start, and then a itself start will be called. And on stop routine a stop called first, then d stop, then both b and c stops parallel.

The rule here is first nesting level is sequential, second nesting level is parallel

API

Here is public methods you get on inheritance/mixin:

wait

async def wait(self):

Wait for service stop. Service must be started. This is useful when you use service as a context manager.

run

async def run(self):

Run service and wait until it stop.

graceful_shutdown_timeout

@property
def graceful_shutdown_timeout(self):
    return 10

How much total time in seconds wait for stop routines. This property can be overriden with subclass:

class CustomServiceMixin(ServiceMixin):
    @property
    def graceful_shutdown_timeout(self):
        return 60

dependencies

@property
def dependencies(self):
    return []

Should return iterable of current service dependencies instances.

running

@property
def running(self) -> bool:

Check if service is running

add_task

def add_task(self, coro) -> asyncio.Task:

Add background task.

start

async def start(self):
    pass

Start routine.

stop

async def stop(self):
    pass

Stop routine.